Surface based electromagnetic (EM) data acquisition is well described in the geophysical literature in published papers and books. The types of EM data acquisition configurations considered below are described in texts such as Kaufman, A. A., and Keller, G. V., 1983, “Frequency and Transient Electromagnetic Sounding”, published by Elsevier and Nabighian, M. N., 1991, “Electromagnetic Methods in Applied Geophysics”, published by the Society of Exploration Geophysicists.
Surface-to-borehole EM data acquisition is described in Dyck, A., 1991, “Drill-Hole Electromagnetic Methods”, pp. 881-930, in Electromagnetic Methods in Applied Geophysics edited by Nabighian, M. N., but he limits his discussion in this text to exploration for metallic ores using simple models and does not apply the technology for subsurface formation imaging.
As discussed in more detail below, sensitivity studies of typical surface to borehole electromagnetics configurations show high sensitivity near both the source and receiver positions. Whereas a high sensitivity is desired near the borehole-deployed receiver array to map the formation resistivity, it is not useful near the surface-deployed transmitter. In addition, the geology near the earth's surface tends to be complex and discontinuous.
Our challenge is to focus the measurement on the target of interest, the near well formation, while satisfying the sensitivity functions near the transmitter. This problem is analogous to the statics issue faced in surface seismics, where the local conditions can statically shift the formation response from the expected state.